A ferritic stainless steel sheet is widely used for applications such as home electric appliances, kitchen instruments, electronic apparatuses and the like. However, a ferritic stainless steel sheet is inferior to an austenitic stainless steel sheet in workability and therefore the applications of a ferritic stainless steel sheet are sometimes limited.
During the course of attempting to solve the above problem, refining technologies have improved recently and therefore it has been possible to reduce carbon and nitrogen to ultra-low levels and, further, by adding stabilizing elements such as Ti and Nb, to improve formability.
The conventional technologies for improving the formability of a ferritic stainless steel sheet are mostly ones for improving deep drawability, namely an r-value. With regard to hot rolling conditions for example, the technologies for improving an r-value by regulating a hot-rolling temperature and so on are disclosed in Japanese Unexamined Patent Publications No. S62-77423 and No. H7-268485. However, the real situation is that, even using such technologies, satisfactory properties are sometimes not secured when the amounts of steel components fluctuate. Further, with regard to cold rolling conditions, the technologies for improving an r-value by applying rolling with large diameter rolls are disclosed, for example, in Japanese Unexamined patent Publications No. S59-083725, No. S61-023720 and No. 2000-178696. Moreover, there have been the cases where satisfactory properties are not secured depending on the steel components, intermediate annealing or final annealing conditions.
Furthermore, in actual working, only deep drawing formability is not enough and punch stretchability is often required. A ferritic stainless steel has the drawback of very poor punch stretchability because it is inferior to an austenitic stainless steel in elongation. However, studies on the drawback have scarcely been done. An improvement in elongation is effective for the improvement of punch stretchability and the technologies related to components for improving punch stretchability are disclosed, for example, in Japanese Unexamined Patent Publication No. S58-061258, No. H01-075652 and No. H11-350090. However, the real situation is that, by technologies which merely adjust steel components, satisfactory elongation, namely satisfactory punch stretch formability, is not secured.
Still further, a problem of a ferritic stainless steel sheet is that linear jogs called ridging appear on the surface thereof after the steel sheet is subjected to press working and, when the ridging is excessive, cracks occur during working. A technology for improving ridging by adjusting hot rolling conditions is disclosed, for example, in Japanese Unexamined Patent Publication No. H04-341521. However, the basic concept of the technology is to accelerate recrystallization by applying large reduction rolling at rough rolling and the drawbacks in this case are that significant defects appear on a hot-rolled steel sheet and also that excessive ridging appears in the event of severe working. In addition, as technologies for improving ridging by fractionizing a solidification structure, the technologies wherein Mg oxide particles are controlled by adding Mg are disclosed in Japanese Unexamined Patent Publication No. H10-324956 and No. 2000-192199. However, the drawback in such disclosed technologies is that ridging occurs unevenly and even excessively in the event of severe working.
Meanwhile, a so-called high-purity ferritic stainless steel wherein the amounts of C and N are lowered and Ti is added as a stabilizing element has a lower possibility of generating stress corrosion cracking than SUS 304, that represents an austenitic stainless steel, and further, it has the advantage of lowering costs because it does not contain Ni. However, a drawback of a high-purity ferritic stainless steel is that the elongation, that is important as an index of workability, is lower than that of SUS 304. Further, for improving the workability of a high-purity ferritic stainless steel, it is necessary to lower the amounts of C and N, as interstitial solid solution elements, and also the amounts of Si, Mn, P, Ti, etc., as substitutional solid solution elements.
When a higher purification of a ferritic stainless steel is further attempted, such a high-purity ferritic stainless steel is liable to develop a coarse columnar crystal structure in the structure of a casting that is the raw material of a steel sheet and to cause roping of a cold-rolled steel sheet and ridging, at the working of a cold-rolled and annealed product, to occur conspicuously. In an attempt to reduce roping and ridging, methods wherein a casting structure is made to be composed of equiaxed crystals and thus the structure is fractionized are proposed. A typical method is to add Ti (about 0.2 to 0.3 mass % for example), precipitate TiN in molten steel before the molten steel solidifies, and then accelerate the formation of nuclei for solidification by using TiN as the nuclei of heterogeneous nucleation (Hidemaro Takeuchi et al, Tetsu To Hagane, 66 (1980) 638). According to this method, when an equiaxed crystal ratio is controlled to about 60 to 70% or more, ridging is effectively reduced. In this method, however, since Ti is added by about 0.2 to 0.3%, Ti exceeding the amount required for the formation of TiN dissolves unavoidably in steel and, resultantly, the elongation of a steel sheet deteriorates. Therefore, the method is not compatible with the intention of improving the workability of a steel sheet.
A method wherein, even with a smaller addition amount of Ti, equiaxed crystallization is accelerated by complexly precipitating TiN in Al—Ti type inclusions has been disclosed (Japanese Unexamined Patent Publication No. 2000-144342). The method makes it possible to prevent the deterioration of the elongation of a steel sheet caused by an excessive amount of Ti. However, Si must be contained for precipitating TiN by this method as explained later. It is well known that Si deteriorates the elongation of a steel sheet even though the addition amount is small. Therefore, in this method too, to make a casting structure composed of equiaxed crystals and fractionized in order to recure ropind and ridging is not compatible with enhancing elongation.
The object of the present invention is, by solving the problems of prior art, to provide a method for producing a ferritic stainless steel sheet excellent in deep drawability, punch stretchability and ridging resistance.
In aforementioned conventional technologies in particular, Ti and Si, that deteriorate the elongation of a steel sheet, must be used inevitably for fractionizing a casting structure and thus reducing roping and ridging. Therefore, the technologies are not compatible with the expectations of highly purifying a steel sheet and thus securing such workability that allows SUS 304 to be replaced with the steel sheet. In view of the above situation, the object of the present invention is to make it possible to secure both a high workability of a steel sheet and the enhancement of roping and ridging resistance simultaneously by reducing to the utmost the amounts of Ti and Si that cause the elongation of the steel sheet to deteriorate and thus attaining the substantial fractionization of a casting structure even when a high purity is maintained.